1. Field of Invention
The present invention relates to a method of increasing the absorption rate of peroxides into polymer particles used to form electric cable insulation.
2. Description of Related Art
Peroxide compounds are commonly used to cross-link (vulcanize) polymers. In some cross-linked polymer compositions, the peroxide compound is introduced into the polymer through a milling process. However, in the preparation of polymer particles used to form electric cable insulation, the peroxide compound is more commonly introduced into polymer particles through an absorption/coating process such as described in Gray et al., U.S. Pat. No. 3,455,752, which is hereby incorporated by reference in its entirety. Use of an absorption/coating process prevents premature decomposition of the peroxide during processing of the polymer particles.
In the absorption/coating process, solid, pre-formed polymer particles (e.g., granules or pellets) that comprise one or more melt-blended polymers and other optional materials (e.g., anti-oxidants, pigments, fillers etc.) are contacted under physical mixing conditions with at least one peroxide compound at a temperature above the melting point of the peroxide compound but below the softening point of the polymer particles. For example, dicumyl peroxide, which has a melting point within the range of 39-41° C., can be contacted with and absorbed into olefin-containing polymer particles using the absorption/coating process at temperatures between about 40° C. and 80° C. Very little of the dicumyl peroxide decomposes at these temperatures, particularly at the lower end of the temperature range.
Once the peroxide compound has been sufficiently absorbed into the polymer particles, the peroxide-containing polymer particles can be used immediately to coat conductor wires or bundles, or they can be permitted to cool to ambient temperatures and stored for later use. The peroxide-containing polymer particles are particularly useful for forming the polymeric insulation layer on electrical power transmission cables. Wire and cable manufacturers typically feed the peroxide-containing polymer particles into a system in which the peroxide-containing polymer particles are melted in a screw feed and extruded coaxially onto a wire conductor or wire bundle. The polymer-coated wires are then passed through a long heated tube called a catenary. Temperatures in the catenary are sufficiently high to cause the peroxide in the extruded polymer to decompose and thereby cross-link the polymer. Cross-linking is necessary for power cable insulation because the cables can become hot during use due to resistive heating and environmental conditions. If the polymeric insulation was not cross-linked, then the polymeric insulation could melt or soften, which could cause the electrical cable to fail.
Although the absorption/coating process is advantageous in that it prevents the premature decomposition of the peroxide, it tends to be a time-consuming process step. The molten peroxide must be in contact with the polymer particles under mixing conditions at elevated temperatures for a significant period of time in order to obtain adequate coating and absorption of the peroxide into the polymer particles. Contact periods from ten minutes to one hour are typical, which makes the absorption/coating process a rate determining step in the fabrication of peroxide-containing polymer particles.
Three factors that are known to affect the rate at which peroxide compounds absorb into polymer particles are: (1) the temperature at which the peroxide compound and the polymer particles are contacted together; (2) the physical characteristics of the polymer; and (3) the molecular weight of the peroxide compound. Higher contact temperatures tend to increase the rate of absorption. More crystalline polymers such as polyethylene tend to absorb peroxides more slowly than relatively amorphous polymers such as ethylene-propylene rubber. And, lower molecular weight peroxide compounds tend to absorb into polymer particles at a more rapid rate than higher molecular weight peroxide compounds.
Unfortunately, it is not commercially feasible to increase the rate of absorption of peroxide compounds in polymer particles by manipulating these three factors. Increasing the temperature at which the peroxide compound and the polymer particles are contacted disadvantageously increases the energy costs associated with the process, increases the risk of premature decomposition of the peroxide compound and might require prohibitively expensive modifications to existing processing equipment. It is also not commercially feasible to alter the composition of the polymers or peroxide compounds used to manufacture polymer particles used to form electric cable insulation because of cost issues and the potential for detrimental changes in electrical insulation performance.